The present invention relates generally to optical communications, and particularly to the splicing of optical fibers in optical communications systems and devices.
In the construction of devices for optical communications, and in the deployment of devices and optical fibers in optical communications systems, it is often necessary to splice two optical fibers to one another, so that an optical signal may be transmitted from one to the other. As the mode field diameters of commonly used optical fibers may be as small as about five microns, the splicing operation must be performed with a high degree of precision in order for an optical signal to travel between the two optical fibers without experiencing an untoward amount of loss. For a low-loss splice, it is necessary for the cores of the two optical fibers to be held in substantial optical and physical contact with one another.
One low-loss method for splicing optical fibers is fusion splicing. In this method, an end of one optical fiber is aligned to be in substantial optical and physical contact with an end of another optical fiber. The ends are generally prepared by cleaving or polishing. The aligned, abutted ends of the optical fibers are heated to a temperature above their glass transition temperatures, causing them to soften and melt together. Upon cooling, the melted together area fuses into a continuous joint. When used to form splices between the same type of fibers, or between fibers with similar therrnomechanical properties, the fusion splicing method works very well, yielding splices with mechanically strong joints and very low optical loss. However, the method is less useful when splicing optical fibers with dissimilar thermomechanical properties. For example, when one optical fiber has a much lower glass transition temperature and a much higher coefficient of thermal expansion than the other, the splice is extremely susceptible to fracture. The instability of the splice is due to the residual mechanical stress formed between the thermomechanically dissimilar materials of the joint upon cooling. The process of fusion splicing optical fibers with widely differing softening temperatures is also difficult. For example, at a temperature sufficient to soften the fiber with the higher softening temperature, the fiber with the lower softening temperature will be significantly distorted due to viscous flow. Lastly, direct fusion splicing may damage heat-sensitive fibers, such as, for example, polarization maintaining fibers.
Passive physical alignment methods have also been used to splice fibers. In general, these methods involve a physical alignment structure holding the ends of two fibers in physical alignment with one another. For example, optical fibers may be held in ferrules, and the ferrules physically aligned by a precision molded sleeve. Alternatively, cleaved ends of two optical fibers may be abutted in a glass sleeve, which is collapsed by softening, thus bringing the fibers into physical alignment with one another. Fibers have also been spliced by placing connectors on the ends of the fibers to be spliced, followed by mating of the connectors. Methods such as these effectively align the outer circumferences of the optical fibers. Thus, satisfactory optical performance ensues only if the cores of both optical fibers are concentric with the outer circumferences of the fibers. This method is not suitable for splicing optical fibers with non-centrosymmetric cross-sections.
One method proven workable for the splicing of dissimilar, non-concentric optical fibers involves the use of an optical path adhesive. In this method, the ends of the optical fibers are aligned, and an optical path adhesive is applied to the area between and around the optical fibers and then cured. However, shrinkage of the adhesive during cure can cause the optical fibers to become somewhat misaligned, increasing the optical loss of the splice. Further, this method requires the design of a robust package that will protect the joint and ensure long-term optical and mechanical reliability.
Accordingly, there remains a need for a method for creating a reliable, low optical loss splice between optical fibers of dissimilar thermomechanical properties and unknown concentricity.
One aspect of the present invention relates to a method of creating an optical fiber splice including the steps of providing a first optical fiber held by a first ferrule and a second optical fiber held by a second ferrule; aligning the end of the first optical fiber to the end of the second optical fiber; and applying energy to the ferrules, thereby sealing the face of the first ferrule to the face of the second ferrule such that the core of the first fiber and the core of the second fiber remained aligned. The ferrules have softening temperatures at least 30xc2x0 C. below the lower of the glass transition temperatures of the optical fibers.
Another aspect of the present invention relates to a method of creating an optical fiber splice including the steps of inserting a first optical fiber into a first ferrule such that the end of the first optical fiber is substantially coplanar with the face of the first ferrule; affixing the first optical fiber to the first ferrule; inserting a second optical fiber into a second ferrule such that the end of the second optical fiber is substantially coplanar with the face of the second ferrule; affixing the second optical fiber to the second ferrule; polishing the faces of the ferrules, whereby slight protrusions of the ends of the optical fibers beyond the planes of the faces of the ferrules is produced; aligning the end of the first optical fiber to the end of the second optical fiber; and applying energy to the ferrules, thereby sealing the face of the first ferrule to the face of the second ferrule such that the core of the first fiber and the core of the second fiber remained aligned. The ferrules have softening temperatures at least 30xc2x0 C. below the lower of the glass transition temperatures of the optical fibers.
Another aspect of the present invention involves a ferrule for use with an optical fiber, the ferrule having a channel dimensioned to receive the optical fiber, wherein the material of the ferrule has a softening temperature at least 30xc2x0 C. less than the glass transition temperature of the optical fiber.
The present invention results in advantages over existing methods of splicing optical fibers. The methods of the present invention are suitable for use in splicing optical fibers of dissimilar size, mode field diameter, and thermomechanical properties. The methods of the present invention are not dependent upon the concentricity of the optical fibers, and are therefore suitable for use with non-centrosymmetric optical fibers. No adhesive is needed in the optical path, obviating concerns about high optical power damage due to the presence of the adhesive. The splice of the present invention is easily formed and is mechanically reliable due to a reduction of residual stress at the joint and the strong interferrule bond. As the person of skill in the art will recognize, the present invention is especially suitable for splicing pairs of optical fibers that are not amenable to typical fusion splicing techniques; for example, multi-component silicate amplifying optical fibers may be reliably spliced to single mode 980 nm pump optical fibers. Further, the method may be advantageously used to splice fibers, such as polarization maintaining fibers, which may be damaged by the intense heat of the fusion splicing process.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to these skilled in the art from the description or recognized by practicing the invention as described in the written description and claims hereof, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework to understanding the nature and character of the invention as it is claimed.
The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate one or more embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.